In natural vitamin A and alkanoyl esters thereof, the five conjugated double-bonds all have the trans-configuration. Of all the isomers, all-trans vitamin A or alkanoyl esters thereof, possess by far the greatest biological activity and are accordingly used almost exclusively in human and animal nutrition. Since present commercial vitamin A preparations are almost exclusively of the synthetic type and since the hitherto known and used processes for the manufacture of vitamin A do not yield pure all-trans compounds, but rather mixtures of various isomers containing more or less large amounts of the all-trans isomer, there has always been the problem of isomerizing the various isomers into the all-trans compound, the isomerization of the 9-cis compound into the all-trans compound never having been satisfactory achieved. The problems which arise are the obtention of the highest possible yields of all-trans compounds and, since the total cis-trans conversion is not possible, the obtention of mixtures from which the all-trans isomer can be isolated in the easiest possible manner. The method most widely used heretofore has been isomerization with iodine in the presence of pyridine. This method is not particularly favorable since only 11-cis and 13-cis as well as 11,13-di-cis isomers, but not 9-cis or 9,13-di-cis isomers can be converted into all-trans compounds. Photochemical isomerization using sensitizers is also known. This method is fraught with disadvantages since, after completion of the isomerization, the sensitizer must be removed. Removal of the sensitizers, especially when working on an industrial scale, can result in considerable difficulties. The irradiation of dilute solutions, in concentrations which are usual in photochemistry, in the absence of sensitizers is also unfavorable for the isomerization since dimers are primarily formed.
Accordingly, the problem to be solved consists in finding and isomerization process whereby there can be obtained from pure isomers or mixtures thereof, a mixture which has the highest possible content of all-trans compound and from which the all-trans isomer can then be isolated readily and in the purest form possible.